Graphene is a substance made of pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite, but in a one-atom thick sheet (hence leading to the oft-used graphene descriptor “two-dimensional”). Graphene is the basic structural element of many carbon allotropes including graphite, charcoal, carbon nanotubes, and fullerenes. Graphene has a number of interesting optical, electrical, and thermal properties and has been the subject of much recent research. For example, the 2010 Nobel Prize in Physics went to researchers working on “groundbreaking” experiments involving graphene.
Recent interest includes exploring the use of graphene in a photovoltaic component such as a so-called solar cell. Note, for example, that a stack of forty-three graphene layers has an opacity of about ninety-nine percent and essentially completely absorbs light throughout the visible and near infrared regions of the solar spectrum notwithstanding having a film thickness of less than fifteen nanometers.
That said, and although a graphene layer having a thickness of only about one ten-thousandth the width of a human hair would in fact absorb essentially all of the light incident upon it, other requirements must be met in order for graphene to function as a useful part of a solar cell. In such a device, the absorption of electromagnetic radiation results initially in the excitation of electrons accompanied by the creation of electron/hole pairs. A corresponding useful behavior is therefore the existence of a potential barrier that impedes excited electrons from recombining with holes. Rather than recombining with the release of energy as heat, electrons and holes in a useful device are preferably enabled to travel in opposite directions, thereby allowing the completion of an electrical circuit from which energy can be extracted as electricity.
The prior art has sought to meet this requirement through the creation of alternating layers of graphene and a material that captures the photo-generated electrons. For example, multilayer hybrid films consisting of alternating layers of graphene and electron-capturing titanium oxide have been realized and characterized for their potential as photovoltaic devices. Those hybrid layers were built up using laborious layer-by-layer procedures that required elaborate, expensive, synthetic methodologies. Accordingly, even though such hybrid films exhibit interesting photovoltaic properties, the corresponding burdens associated with their synthesis discourages those skilled in the art from seriously considering such films as candidates in a commercial application setting.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. Certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. The terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.